1. Field of the Invention
This invention relates to a resin composition with improved mechanical characteristics, and more particularly to a resin composition with improved mechanical characteristics that comprises an aromatic polyester-polycarboxylic anhydride and a polyamide.
2. Description of the Prior Art
An aromatic polyester copolymer (hereinafter, referred to as APE) prepared from aromatic dicarboxylic acid or functional derivatives thereof and a diphenol or functional derivatives thereof is a heat resistant, thermoplastic resin having high heat distortion temperature and high heat decomposition temperature as well as good mechanical and electrical properties. It is widely used in fields where engineering plastics of high quality are required. However, because of its high heat distortion temperature, it is less moldable than other thermoplastic resins; for instance, making an injection molded article of it requires high molding temperature and injection pressure whereas the resulting molded article often develops a sink mark, flow mark or excessive internal strains.
Molded articles with sink marks, flow marks or excessive internal strains are not only poor in appearance but are greatly modified in mechanical properties so it frequently occurs that the excellent performance of the resin from which they are molded will not be exhibited to the fullest. Depending on the kind of molded article, the flow length is sometimes insufficient to provide a perfect product. Therefore, it has been required that good engineering plastics be excellent not only in their various physical and mechanical properties but in moldability also. However, it is also known well that the use of a polymer blend to improve the moldability of a heat resistant engineering plastic tends to decrease its heat distortion temperature. Furthermore, while APE has the advantages mentioned above, it can only be used in limited applications since it is not resistant to many organic solvents.
Polyamides are very excellent in their resistance to organic solvents, wear resistance and moldability, but they are not adequately heat resistant as typified by polycaprolactam and polyhexamethylene adipamide which have a heat distortion temperature of 56.degree. C. and 59.degree. C., respectively. As a further disadvantage of polyamides, they have high hygroscopicity and are known to be subject to dimensional variation, changes in mechanical performance and reduction in electrical properties.
A resin composition comprising APE and a polyamide has considerably higher heat distortion temperature than a polyamide as well as higher moldability and solvent resistance than APE and the improvement exceeds what might be expected from the additive effect based on the proportion of the two polymers that constitute the resin composition. Therefore, the resulting composition has very useful properties in that it retains not only the excellent tensile strength, bend recovery characteristics and dimensional stability of APE but the high crack resistance and wear resistance of the polyamide [see Japanese patent application (OPI) Nos. 4146/75 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) and 98765/77].
It turned out, however, that articles made, for example, by injection molding of a resin composition comprising APE and polyamide had the following defects: The molded article easily breaks at a thin-walled portion such as a gate or at a long narrow rod-like projection especially when it is absolutely dry. Thus limits are imposed on the practical applications of the polymer such as its release from a mold as well as product design. What is more, even a relatively thick-walled article easily cracks upon impact such as a drop impact and a falling-ball impact especially when it is absolutely dry. All these phenomena constitute a considerable limitation on the usefulness of such resin compositions.
Japanese Patent Publication No. 879/69 describes a method of reacting a mixture of an aromatic dicarboxylic acid and a diphenol with an aromatic dicarboxylic acid chloride to produce an aromatic polyester-polycarboxylic anhydride having ester linkages and a carboxylic acid linkage in the main chain and the use of such an aromatic polyester-polycarboxylic anhydride in the production of a fiber or film.